There is a continuing interest in personal communications systems, such as cellular telephones and pagers. Product requirements for these systems typically call for very small, lightweight, and low cost antennas. Microstrip antennas have been used in personal communication systems to accommodate these smaller design requirements, because they can be fabricated using inexpensive printed circuit board technology. Over the years, many forms of microstrip antennas have been developed, the "patch" antenna being one of the most popular. Patch antennas typically comprise radiator elements in the form of rectangular or square patches disposed onto a substrate over a ground plane. The substrate materials used for patch antennas typically have dielectric constants (.beta..sub.r) below 10 in order to achieve wider bandwidths. However, the major weakness of microstrip antennas still remains their very narrow impedance bandwidth characteristics.
FIG. 1 shows a prior art patch antenna 100 formed with a single rectangular patch having a resonant length (along length 110) characterized by equation: ##EQU1## c is the speed of light, f is the resonant frequency, and .epsilon..sub.r is the dielectric constant of substrate. To improve the bandwidth of this single resonant circuit, additional patches can be added to provide two resonant circuits. FIG. 2 shows a prior art patch antenna 200 with two gap-coupled rectangular patches 202, 204. The advantage of the two gap-coupled rectangular patches over the single patch is an increase in bandwidth, however the disadvantage is that the gap-coupled rectangular patches require an increase in the overall size of the antenna to achieve the improved bandwidth.
As an example, a single patch antenna, such as the antenna shown in FIG. 1 (not to scale), can be designed to resonate at a frequency of 1.85 gigahertz (GHz) when formed on a ceramic filled polytetrafluoroethylene (PTFE) substrate 102 having a dielectric constant .epsilon..sub.r =6. Substrate dimensions measuring 4.4 centimeters (cm) along width 104, by 3.7 cm along length 106, with a patch size measuring 3.8 cm along width 108, by 3.1 cm along length 110 produce a bandwidth of approximately 13.8 megahertz (MHz). The bandwidth can be increased by providing a longer substrate, such as the antenna shown in FIG. 2 (not to scale), measuring 6.9 cm along length 206 and with the second patch 204 having the same width but a slightly longer length 208 of 3.15 cm. With this second configuration the bandwidth increases to approximately 78 MHz, but the size of the antenna structure has effectively doubled. Increasing the size of the antenna structure by adding multiple patches thus makes an antenna less attractive for use in portable communications equipment which is troublesome since small size is particularly desirable in hand-held products, such as cellular handsets. Accordingly, there is a need for an improved microstrip antenna which provides a small, light weight, cost effective structure.